I got the following error when trying to "give" a literal natural number in the Emacs UI:

  *** Exception: src/full/TypeChecking/Substitute.hs:35: the impossible happened

This refers to the Apply instance for Term:

 instance Apply Term where
     apply m [] = m
     apply m args@(Arg _ v:args0) =
       case m of
	    Lit l -> __IMPOSSIBLE__

I have not found a small example reproducing the bug (but I have not tried very hard).

Fixed. -- Ulf